You get the oil changed in your car regularly, why shouldn't you tune up your computer?
That's the pitch of iolo technologies, maker of the software utility System Mechanic, which has been used on roughly 21 million computers to date (but not, alas, Macintosh ones, which means I can't try it out on my 4-year-old PowerBook). If one of your year-end green tech resolutions involves eking even just six months more life out of your existing systems without compromising performance, the relatively new, 64-bit version of System Mechanic could make that goal a little easier to accomplish.
Iolo is pitching its software utility as a way of helping keep your Windows Vista, XP or 2000 system running reliably and efficiently for longer than the average system's lifespan of about 2.4 years (as estimated by the Environmental Protection Agency). How long it lasts really depends on what you need it to do. When you think about it, the main reason you should update your computer is because there has been some catastrophic software failure, not because the software frustrates you. In theory, the latter can be addressed. We might want to start setting some new standards for what constitutes a "totalled" computer.
Two big things you'll really care about in the System Mechanic product line: ActiveCare 2.0 (in the $50-ish System Mechanic Professional), which can sense when you're not using your system so that it can do its job in terms of defragmentation and compression. You don't have to set it to run at any specific time, it takes advantage of when your computer is on but idle. The other thing I'd like to point out for you frugal small-business owners is that each package of System Mechanic entitles you to run the utility on up to three systems.
If you've decided that your PC is definitely on its last legs, iolo is also the developer of DriveScrubber, which helps you ensure that all traces of confidential data have been removed from the computer. A cool feature of this utility is that it will allow you to clear out all the data files associated with an application while enabling you to keep the original application on the computer, which can help with the trade-in/asset recovery/recycling value associated with a computer.